Tuesday, July 26, 2016

Electromagnetic radiation and its characteristics

Electromagnetic radiation: Electromagnetic radiation (EMR) is a a form of energy propogated through free space (vaccum) or a medium in the form of electromagnetic waves.EMR is termed as such because it is composed of an electric field and a magnetic field that oscillate simultaneously in planes mutually perpendicular to each other as well as to the direction of propogation of the radiation.

The two defining characteristics of electromagnetic radiation are its:

  1. Frequency and
  2. Wavelength
Frequency is the number of waves that pass a point in a specified time. It is measured in Hertz (Hz) or cycles per second.
Wavelength is the distance between two successive peaks of a wave. It is measured in meters (m) or its multiples (nm, mm, cm etc)

The range of electromagnetic waves is called electromagnetic spectrum.

(Velocity of light (C)~ 3*10^8 m/s)
Velocity, wavelength and frequency are related by the equation: C = frequency * wavelength
It is evident from this equation that frequency and wavelegth are inversely proportional
This follows that:
  • a wave with a longer wavelength has lower frequency and thus lower energy
  • a wave with a shorter wave wavelength has higher frequency and thus higher energy.
The electromagnetic spectrum is divided into seven regions. They are:
  1. Radio waves
  2. Microwaves
  3. InfraRed (IR) waves
  4. Visible light
  5. UltraViolet (UV) rays
  6. X rays and
  7. Gamma rays
Usually. low energy radiation (Radio waves) is expressed as wavelengths while microwaves, infrared (IR), visible and ultraviolet (UV)  radiations are expressed as frequencies.
Radio waves
Radio waves are at the lowest range of the EM spectrum, with wavelengths greater than about 10 mm. Radio is used primarily for communications including voice, data and entertainment media.

Microwaves have wavelengths of about 10 mm to 100 micrometers (μm). Microwaves are used for high-bandwidth communications, radar and as a heat source for microwave ovens and industrial applications.

Infrared is in the range of wavelengths of about 100 μm to 740 nanometers (nm). IR light is invisible to human eyes, but we can feel it as heat if the intensity is sufficient.

Visible light
Visible light is found in the middle of the EM spectrum, between IR and UV. It has wavelengths of about 740 nm to 380 nm. Visible light is defined as the wavelengths that are visible to most human eyes.

Ultraviolet light is in the range of the EM spectrum between visible light and X-rays. It has wavelengths of about 380 nm to about 10 nm. UV light is a component of sunlight; however, it is invisible to the human eye. It has numerous medical and industrial applications, but it can damage living tissue.

X-rays are roughly classified into two types: soft X-rays and hard X-rays. Soft X-rays comprise the range of the EM spectrum between UV and gamma rays. Soft X-rays have wavelengths of about 10 nm to about 100 picometers (pm). Hard X-rays occupy the same region of the EM spectrum as gamma rays. The only difference between them is their source: X-rays are produced by accelerating electrons, while gamma rays are produced by atomic nuclei.

Gamma-rays are in the range of the spectrum above soft X-rays. Gamma-rays have wavelengths of less than 100 pm (4 × 10−9 inches). Gamma radiation causes damage to living tissue, which makes it useful for killing cancer cells when applied in carefully measured doses to small regions. Uncontrolled exposure, though, is extremely dangerous to humans.

Maps - Basic components, Types of maps & Map analysis

Basic components of a map:
The basic components of any map are listed below:

  1. The Title of the map indicates what the map is trying to show
  2. The Key explains the symbols shown in the map
  3. The Scale gives the relationship between distance on the map to the actual distance on the Earth.
  4. Tha map shows the Latitudes (parallels N or S of the equator) and Longitudes (meridians E or W of the prime meridian)
  5. Compass rose showing the directions on a map.
The types of maps are:
  1. Physical map
  2. Political map
  3. Thematic map
  4. Cartogram and
  5. Flow-line map

Map Analysis involves answering questions based on:
  1. Title of the map
  2. Type of map
  3. Location of an object, area or phenomena
  4. Meanings of the symbols and inferences from patterns
  5. Relationship between locations and events over a period of time
  6. The main idea or theme being represented by the map.

GIS-Unit 5-Syllabus-OU


Introduction to remote sensing: Electromagnetic radiation, Characteristics, Interaction with Earth's surface, Sensor types, Satellite characteristics, IRS series, Data products, Interpretation of data.

Software Scenario Functions: Watershed modelling, Environmental modelling and Visibility analysis.

Map transformations


Map transformation involves transformation of points from one map to points on another map while minimizing the differences between the two sets of points.

  • The 'Helmert transformation' is the best choice for the vast majority of applications.
  • The 'Helmert transformation' translates the points of one map horizontally and vertically and also rotates and scales the points (it uses 4 parameters).
  • The 'affine transformation' is useful in cases where the paper has pronounced directional shrinking due to orientation of the fibers.
  • The 'affine transformation' is also useful to compensate for some shearing in the map or for computing the shearing angle.
  • The 'affine transformation' with five parameters translates points in the x direction and y direction, a rotation and two scale factors (one in the x direction and one in the y direction)
  • The 'affine transformation' with six parameters consists of:
    • translation in x direction
    • translation in y direction
    • two rotation and two scale factors (both axes are rotated and scaled separately)
Other map transformations used are:

  1. Robust Helmert
  2. Huber estimator
  3. Vestimator and
  4. Hampel estimator